docs: def key_type(self, type_):
          """returns reference to the class key type declaration"""
          if not self.is_mapping(type_):
              raise TypeError(
                  'Type "%s" is not "mapping" container' %
                  str(type_))
          return self.__find_xxx_type(
              type_,
              self.key_type_index,
              self.key_type_typedef,
              'container_key_type')